In recent years, vehicle-to-vehicle communication devices are used to determine vehicles that may possibly be affecting a subject vehicle's traveling condition by exchanging driver's intention in both ways between the subject vehicle and the affecting vehicle. For example, a conventional vehicle-to-vehicle communication device disclosed in Japanese Patent Document JP-A-2002-183889 captures a driver's voice indicating that the affecting vehicle should be stopping for a short time, and transmits a stop request to the affecting vehicle, when the subject vehicle having the vehicle-to-vehicle communication device enters into a traffic on a road from a shoulder of the road. More practically, the vehicle is determined by the device on the subject vehicle as possibly colliding with the subject vehicle. The device on the affecting vehicle having received the stop request displays positions of the affecting vehicle and the subject vehicle requesting a stop on a display of the device to inform the driver of the affecting vehicle of the stop request.
If a driver of the affecting vehicle responds to the stop request by indicating an acknowledgement by voice, the device on the affecting vehicle sends back the acknowledgement to the device on the subject vehicle. The device on the subject vehicle entering the traffic with the acknowledgement having received from the affecting vehicle also displays the positions of the subject vehicle and the affecting vehicle on the display of the device to inform the driver of the subject vehicle of reception of the acknowledgement of the stop request.
However, the vehicle-to-vehicle communication device is not necessarily installed on all of the vehicles on the road. That is, the driver of the subject vehicle cannot get hold of the position of the affecting vehicle by the communication between the devices if the affecting vehicle does not have the device. For example, in a situation having three vehicles A, B and C involved, where the device-equipped vehicle A is stopping at a shoulder of a main street for entering traffic on the main street from a side road, and the device-equipped vehicle B and the device-not-equipped vehicle C are traveling on the main street, a driver of the vehicle A can only get hold of the precise position of the vehicle B by using the device, and can recognize the position of the vehicle C only by sight if it exists in a viewable area of the driver of the vehicle A.